The capacity of a wireless communication link, also referred to herein as a radio link, can be doubled by combining two independent wireless transmitters through an Orthogonal Mode Transducer (OMT), transmitting channel carriers on orthogonal Horizontal (H) and Vertical (V) polarizations, and separating the H and V channels at a receive antenna with another OMT. The receive OMT connects to two separate radio receivers. Because of non-ideal OMT performance and properties of the propagation path, for example, there is some cross-coupling of the channels, which in turn causes cross-channel interference. Herein, “channel” is intended to convey the notion of one or the other of a pair of orthogonal polarization paths including transmitter, transmit antenna and OMT feed, propagation medium, receive antenna and OMT feed and the receiver, both channels operating on one nominal carrier frequency. When referring to a frequency channel as determined by a carrier frequency, the distinction from an orthogonal channel is made by using the term “frequency channel”.
A Cross Polarization Interference Cancellation (XPIC) system can be used to cancel cross-channel interference. An adaptive equalizer algorithm, for example, could detect a correlation between the error caused by the interference on one receiver and the signal received at the other receiver, and generate an interference cancelling signal. If the two transmitters are independent, then the orthogonal channel carriers will have slightly different frequencies and phase noise sources. When the H receiver is synchronized with the H transmitter frequency and phase noise, the interference from the V transmitter appears with a frequency offset and phase noise that results from the frequency and phase noise differences between the two transmitters. The V receiver will see interference from the H transmitter with the inverse frequency offset and phase noise.
Acquiring and tracking this frequency offset can present challenges. An interference signal is generally weaker than a main received signal, and therefore tends to have a very low signal to noise ratio. A frequency offset tracking loop in an XPIC algorithm would then have to have a narrower bandwidth than a carrier recovery loop that is used in demodulating the main received signal. However, this significantly limits the ability of an XPIC algorithm to track both frequency offset and phase noise of interference signals.
Previous attempts to do this by rapid tracking of interference phase in each channel, such as in U.S. Pat. No. 7,046,753, supplement the intrinsic but necessarily slow ability of the XPIC control loop to follow the interference phase. It was subsequently found necessary to further control the parameters of this phase-tracking arrangement by devising an arrangement of master-slave phase-locked loops (PLLs) in each channel, as evidenced in U.S. Pat. No. 7,613,260.
Prior XPIC systems also sometimes suffer from a “dead-zone” in their Error-Vector Magnitude (EVM), where EVM before interference cancellation is not large enough to lock a PLL which tracks its phase, but is large enough to cause data errors.